For producing 4-nitrosodiphenylamines from diphenylamines, the following processes are known: (1) a process comprising adding a solution of hydrogen chloride in an alcohol to a mixture of an aromatic solvent containing a small amount of water, a nitrite salt and a diphenylamine (U.S. Pat. No. 4,362,893); (2) a process comprising reacting diphenylamine with an alkyl nitrite and anhydrous hydrogen chloride in a substantially anhydrous C.sub.5 -C.sub.10 aliphatic alcohol solvent free of an aromatic solvent (Japanese Examined Patent Publication (Kokoku) No. Sho 60-42233); (3) a process comprising reacting diphenylamine with nitrosyl chloride and anhydrous hydrogen chloride in a C.sub.1 -C.sub.10 aliphatic alcohol solvent (Japanese Unexamined Patent Publication (Kokai) No. Hei 2-3631).
However, in process (1), inorganic salts such as nitrite salt and the like always exist in the state of slurry of poor dispersion due to the use of the nitrite salt in the presence of a small amount of water, so that the reaction between solid and liquid becomes inevitable. Furthermore due to the necessity of controlling the amount of water within a certain range in the solvents, the water content needs be controlled if the solvents are recovered for recycling. In process (2), since the alkyl nitrite to be used as an agent for preparation of nitroso compounds is explosive and decomposable, the storage or preservation thereof is not easy. In process (3), nitrosyl chloride to be used as an agent for preparation of nitroso compounds is an extremely corrosive gas and requires equipment for its generation. Therefore none of the processes reached the satisfactory level for the industrial application.